During my IMC my instructor chose a day where conditions were such that the cloudbase was around 3000ft, as was the icing level. He took us up into the cloud while practising some holds, with the purpose of demonstrating what it looks like when ice starts building up on your wings (and temp. probe). We could see how it was gradually building up, then we descended and watched it all fall off. I believe this was not at all dangerous, and Im thankful for having experienced first hand what it's like to see ice building up. I've only experienced it once since in IMC on a cross country, while trying to climb through the cloud instead of staying in it at just below the freezing level. It was a good lesson, and I'm glad of it!